1. Field of the Invention
The present invention relates to a sheet material conveying apparatus for conveying sheet material one by one by separating them from those stacked and held. The invention also relates to a recording apparatus that records on the sheet material serving as a recording medium, and a recording system for controlling the recording apparatus as well.
2. Related Background Art
Conventionally, and generally, there have been in wide use the ink jet recording apparatus, the laser beam printer, and the like, which perform recording by means of ink jet recording process or electronic photographing process by conveying only one sheet of recording medium separated from a stack of plural sheets after being set on a sheet-feeding tray or a sheet feeding cassette.
Also, along with the advancement of the ink jet recording process or the electronic photographing process, it has become possible to provide the output in as high quality as almost a photograph.
On the recording medium side, too, various and many kinds of specially treated medium are available on the market, such as the one having an ink acceptance layer on the surface of the medium in order to make clear color reproduction or having a glossy layer in order to produce the sense of glossiness.
On the other hand, the copying sheet used mainly for text recording or the like, that is, the so-called normal medium, is still in wide use in terms of quantity.
To meet the requirement for the convenient use of a recording apparatus, there exist recording apparatuses capable of handling both the specially treated medium and the normal medium overwhelmingly.
As the separation mechanism for separating and feeding these various and many kinds of recording medium one by one, the following typical ones are available:                1) Nail separation        2) Inclined plane separation        3) Frictional separation        4) Retard separation.        
The nail separation listed as Item 1) above is arranged to perform the separation by bending the edge portion of medium so as to enable it to get over the nail. Therefore, if the medium has thickness beyond a certain extent, a large conveying power is required for bending it. In the sense of practicability, therefore, this type is mainly used for separating the normal medium.
The inclined plane separation listed as Item 2) above is arranged to perform the separation by favorably setting the relations of the magnitude difference among the resistance of the recording medium when it passes the inclined plane upward, the friction between one sheet of medium and another, and the conveying power given to the medium for the intended separation.
The frictional separation listed as Item 3) above is a method for preventing double feeding by use of a member having higher friction coefficient than that of the recording medium, which is called a separation pad (friction pad). This method depends on the friction force between the separation pad and recording medium in order to interrupt the progress of the recording medium for the prevention of double feeding, which makes it difficult to select a separation pad effectively usable for various and many kinds of recording medium. Moreover, the friction force between the separation pad and medium tends to vary depending on the environmental conditions. Thus, there is a limit to the reliability of separation performance.
The retard separation listed as Item 4) above is formed by a driving roller that feeds a recording medium; a driven roller, which is in contact with the driving roller under pressure exerted by a constant load to follow the rotation thereof; and a torque limiter that provides the driven roller with the torque of rotational load when it rotates in the conveying direction of recording medium. In accordance with this method, the torque of rotational load generated by the torque limiter contributes to interrupting the progress of recording medium for the prevention of double feeding. In other words, the force needed for holding the recording medium back, which is converted from the torque of the rotational load provided by the torque limiter, is set at a value larger than the friction force between adjacent mediums when plural sheets thereof are put into the gap between the driving roller and the driven roller. Thus, those sheets of medium are not allowed to pass as they are in overlapped condition. The sheets are made apart between them, and only one sheet is separated from others.
The superiority of this method is that the force needed for holding the medium back can be controlled constantly by means of the torque of rotational load provided by the torque limiter. Therefore, unlike the inclined plane separation or the frictional separation, there is no difference resulting from the kinds of medium as to the resistance when being separated. Also, among some others, this method has an advantage that it is not easily influenced by the environmental conditions.
Now, in conjunction with FIG. 8, the description will be made of the conventional example of the retard type feed and separation device.
This sheet-feeding and separation device conveys a medium 102 to an image-forming portion (not shown). The sheet-feeding and separation device 101 is provided with a pickup roller 105 that feeds and conveys a medium 102 on the uppermost layer one by one from the storing device 103 where the plural number of mediums 102 is stacked; a driving roller 106 that conveys the medium 102 thus fed and conveyed by the pickup roller 105 to the image-forming portion (not shown); the retard roller 107, which is positioned to face the driving roller 106 and separate one sheet of medium by rotating in the direction opposite to the driving roller 106 when the fed and conveyed medium 102 is in plural sheets; and a conveying roller pair 9 arranged in front of the image-forming portion. Also, for the area where the recording medium passes, guides 111 and 112 are arranged to guide the medium 102.
The driving roller 106 and the retard roller 107 are driven by the driving power transmission device 113, which is shown in FIG. 9. The driving power transmission device 113 comprises the driving roller shaft 115, which axially supports the driving roller 106; the retard roller shaft 116, which axially supports the retard roller 107; and the retard roll driving shaft 117, which is connected to the retard roller shaft 116. The retard roller shaft 116 is supported by a supporting member (not shown), which swings freely, to be able to be in contact with or away from the driving roller shaft 115 in parallel thereto. Also, a coupling 119 and a torque limiter 120 are arranged between the retard roller shaft 116 and the retard roller-driving shaft 117. Further, at the end portion of the driving roller 115, there is arranged a clutch 122 to transmit the driving power, which is transmitted from a driving source (not shown) through a driving belt 121, to the driving roller 115. Also, between the driving roller 115 and the retard roller-driving shaft 117, a retard-driving belt 123 is tensioned around to transmit the driving power, which is transmitted from the driving roller 115, to the retard roller-driving shaft 117. In this respect, even if the retard roller 107 is displaced, the coupling 119 exists in order to transmit the driving power from the retard roller-driving shaft 117 to the retard roller shaft 116.
When the medium 102 is conveyed by the driving transmission device 113 one by one in the sheet-feeding direction (direction indicated by arrow b in FIG. 8 and FIG. 9), the retard roller 107 follows the rotation of the driving roller 115 in the direction opposite to the direction of the rotational driving of the retard roller driving shaft 117, because the torque limiter 120 is idle in rotation.
Also, when plural sheets of medium 102 are put into feeding, the torque limiter 120 is not idle, and the retard roller 107 rotates in the same direction of the direction of the rotational driving of the retard roller driving shaft 117, because the friction force exerted between plural sheets of medium is smaller than the friction force between the retard roller 107 and the medium 102. In this manner, only the uppermost medium is separated from the plural sheets of medium 102 put into feeding.
Here, as described above, the transmission of driving power to the retard roller 107 is not necessarily needed. Only with the structure having the torque limiter 120, the separation device can be formed.
Next, dynamically, the description will be made of the conditions that may satisfy the sheet feeding and separation of recording medium 102 in the sheet-feeding and separation device 101 structured as described above. Here, the symbols used for this description will be defined as follows:
Symbols
μ1•2: friction coefficient between paper sheets of the 1st and 2nd sheets
μ2•3: friction coefficient between paper sheets of the 2nd and 3rd sheets
μB•c: friction coefficient between the backside of paper sheet and the retard roller (c)
μF•a: friction coefficient between the front side of the paper sheet and the feed roller (a)
μF•b: friction coefficient between the front side of the paper sheet and the feed roller (b)
P: the pressing force of the retard roller
N: the pressing force exerted by the pressure plate to the feed roller
Tc: the free-running torque of the retard roller (the maximum torque generated)
rc: the radius of the retard roller
FIG. 10 is a view that shows the dynamic model in the case where only one sheet enters the separating portion. FIG. 11 is a view that shows the dynamic model in which two sheets enter the separating portion. In FIGS. 10 and 11, the recording surface (the surface side) is on the upper side, and the backside is on the lower side, respectively.
In FIG. 10, the following two conditional expressions are established:
The condition under which the first sheet is conveyed:μF•b×P+μF•a×N>Tc/rc+μ1•2×NIn other words,P>Tc/(μF•b×rc)−N×(μF•a−μ1•2)/μF•b  (1)
The condition under which the retard roller (c) generates the free-running torque without sliding:μB•c×P>Tc/rcIn other words,P>Tc/(rc×μB•c)  (2)
Also, in FIG. 11, the following separating conditions are established:
The balanced force exerted on the second sheet:μ1•2×P+μ1•2×N=μ2•3×N+Rc  (3)
The condition under which the retard roller (C) stops:Rc>Tc/rc  (4)
The condition under which the first sheet is separated from the second sheet in accordance with the expressions (3) and (4):μ1•2×P+μ1•2×N<μ2•3×N+Tc/rc  (5)
In the expression (5), given μ1•2=μ2•3, the conditional expression becomes as follows:
Separating condition:μ1•2×P<Tc/rcIn other words,P<Tc/(rc×μ1•2)  (6)
FIG. 12 is a view that the aforesaid conditions (1), (2), and (6) are represented in the form of a graph with the pressing force P of the retard roller 7 and the free-running torque Tc of the torque limiter 20 being given as parameters.
In FIG. 12, the portion indicated by slanted lines is the normal feed area. From the representation of FIG. 12, it is understandable that the normal feed area becomes wider if the sheet-feeding conditions are set under the directional condition (toward the upper right in FIG. 12) in which both the pressing force P of the retard roller and the free-running torque Tc of the retard roller are made larger.
However, the recording apparatus provided with the conventional retard roller-separation described above is encountered with the following drawback:
Firstly, as described above, the normal feed area becomes wider if the sheet-feeding conditions are set under the directional condition (toward the upper right in FIG. 12) in which both the pressing force P of the retard roller and the free-running torque Tc of the retard roller are made larger.
However, under such condition, it becomes easier for the recording medium to promote the surface friction of the retard roller, and the retard roller tends to suspend the generation of the free running torque in a short period of time, which may bring about the drawback that the normal feed is disabled eventually.
Secondly, in order to avoid the drawback described in the preceding paragraph, the sheet-feeding conditions are set under the condition in the direction (toward the lower left in FIG. 12) in which the pressing force P of the retard roller and the free-running torque Tc of the retard roller are both made smaller. Here, as long as the friction coefficient between each roller and the medium is significantly larger than the friction coefficient between the adjacent mediums themselves, there occurs no problem. However, in the case of glossy sheets where the difference in friction coefficient is very small between those between each roller and the medium, and the mediums themselves, it is inevitable to use the narrower area of the normal feed area shown in FIG. 12, which is fundamentally narrow as a whole. As a result, when being fed, double feeding tends to occur eventually. Particularly, in the case of the ink jet recording apparatus where it is necessary to feed and convey various and many kinds of medium, this harmful effect may take place often.
As described above, it is very difficult to set the sheet-feeding condition under which all the kinds of mediums can be fed and conveyed stably, and the condition should be set at the sacrifice of some other medium, one kind or another.